PDCCH Order – Network Initiated RACH in New 5G



In 5G NR, PDCCH order is a mechanism using which gNB can force the UE to trigger the RACH procedure. There are  number of reason why UE can triggered the RACH and PDCCH Order is one of them. gNB take PDCCH Order decision when it detect the UE is out-of-sync in Downlink and Downlink data is available for the UE.

In out-of-sync scenario, the gNB triggers a PDCCH order by sending a DCI Format 1_0 on the SSB beam index UE is camped along with a PRACH preamble and RACH occasion. If the gNB find that there is DL Data to be sent to UE in its MAC Buffer and there is a UL synchronization issue due to the expiry of the Time Alignment timer at UE, then the gNB triggers a PDCCH order in order to re-synchronize with the UE.

Telco sought Key Pointers

  • PDCCH order mechanism is available in both LTE and 5G-NR
  • gNB triggers a PDCCH order by sending a DCI Format 1_0 on the SSB beam index UE is camped
  • When UE is configured with two or more Uplink secondary Carriers then 3GPP specifies that the Random Access procedure on an SCell shall only be initiated by a PDCCH order with ra-PreambleIndex different from 0b000000
  • When there is an ongoing RACH procedure triggered by a PDCCH order while the UE receives another PDCCH order indicating the same RACH Preamble, PRACH mask index, and uplink carrier, the RACH procedure is considered as the same RACH procedure as the ongoing one and not initialized again

PDDCH Order Call Flow Sequence

    • gNB detects UE is Out-of-Sync
    • gNB sends PDCCH order to UE with dedicated RACH premable
    • PDCCH order is sent via DCI Format 1_0 with C-RNTI CRC scrambled, containing SSB index, ra-PreambleIndex  and  PRACH Mask Index
    • In the Case of 2-step RACH procedure PDCCH order contains, containing SSB index, ra-PreambleIndex  and msgA-SSB-SharedRO-MaskIndex
    • UE triggers RACH procedure with the information received
    • If no dedicated RACH preamble was sent by network then UE will trigger CFRA procedure and send C-RNTI MAC CE in msg3 for contention resolution
    • UE Completes RACH procedure
    • Now gNB Reconfigures all the IE’s to UE
  • DCI Format 1_0: It is typically used to schedule PDSCH to the UE, DCI format 1_0  can be  CRC scrambled by C-RNTI or CSRNTI or MCS-C-RNTI or P-RNTI or SI-RNTI or RA-RNTI. For DCI Format 1_0 CRC Scrambled by C-RNTI this can be used for normal PDSCH Scheduling by the UE. When PDCCH order is triggered with C-RNTI the network sets the Field “Frequency-domain resource assignment” in DCI with all ones. When UE decodes the DCI Format 1_0 with CRC scrambled by C-RNTI and the “Frequency domain resource assignment” field are of all ones, the DCI format 1_0 is for random access procedure initiated by a PDCCH order UE then decodes below DCI Format 1_0 Fields

  • Random Access Preamble index: This field contains 6 bits. It indicates which RACH preamble to use in case of Contention Free Random Access (CFRA)  or the value 000000 in the case of  Contention based Random Access (CBRA) procedure. If the Preamble index bit is set as ‘0’ then the UE will trigger a contention-based random access procedure or else if the Preamble index is >0 then UE will trigger a Contention-free RACH procedure. Non-zero values are used to allocate the dedicated Prach index (0 to 63) to the UE.
  • UL/SUL indicator: This field contains 1 bit. If the UE is configured with supplementaryUplink  in ServingCellConfig in the cell, this field indicates which UL carrier in the cell to transmit the PRACH according. The Below table indicates the Value of UL/SUL indicator.

  • SS/PBCH index: This field contains 6 bits. This field indicates the SS/PBCH that shall be used to determine the RACH occasion for the PRACH transmission.
  • PRACH Mask index: This field contains 4 bits. If the value of the “Random Access Preamble index” is not all zeros, this field indicates the RACH occasion associated with the SS/PBCH indicated by “SS/PBCH index” for the PRACH transmission. PRACH Mask index is applicable if the preamble index bit is not ‘0’ i.e. a dedicated Preamble index has been assigned by the network to the UE.

  • Reserved bits: 12 bits for operation in a cell with shared spectrum channel access; otherwise 10 bits.

References :

    • 3GPP TS 38.321 NR; Medium Access Control (MAC) protocol specification
    • 3GPP TS 38.214 NR; Physical layer procedures for data

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